It is preferable to design an engine for a constant load so that a desired torque can be generated at a low specific fuel consumption. However, it is difficult to design an engine for driving vehicles so as to have a low specific fuel consumption within the entire range of the engine operation, since load on the engine varies in a wide range.
FIG. 1 shows a fuel consumption characteristic of an engine for a vehicle at various specific fuel consumptions (g/ps.hr), in which the abscissa is engine speed (r.p.m.), and the ordinate is engine torque. Curve A shows the running load (resistance) of a vehicle on a flat road. The curve A is determined by the drag of the body of the vehicle and the gear ratio of the transmission of the engine and the specific fuel consumption is determined by the performance of the engine. It is desirable to design the engine so that the curve A passes through low fuel consumption zones.